1. Field of the Invention
This invention relates to the chemical treatment of lignocellulosic materials. More particularly, the invention relates to the treatment of wood and its derivative materials to obtain increased dimensional stability while improving resistance to biological attack.
2. Description of Related Art
Although wood possesses many unique and desirable properties, it has several undesirable properties which have limited its use for many applications. The physical and chemical properties of wood are the result of the chemistry of the cell wall polymers both individually and collectively.
Wood changes dimension with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding. That moisture swells the cell wall, and the wood expands until the cell wall is saturated with water. Water, beyond this point, is free water in the void structure and does not contribute to further expansion. This process is reversible, and shrinkage occurs as moisture is lost.
Wood is biologically degraded because organisms recognize the polysaccharide polymers in the cell wall and have very specific enzyme systems capable of hydrolyzing these polymers into digestible units. Because high molecular weight cellulose is primarily responsible for strength in wood, strength is lost as this polymer undergoes biological degradation through oxidation, hydrolysis, and dehydration reactions.
Because dimensional instability and biological degradation are chemical phenomena, it is possible to improve both of these undesirable properties of wood by changing the basic chemistry of the cell wall polymers. By chemically modifying the cellulose and hemicellulose components, for example, the highly specific biological enzymatic reactions cannot take place because the chemical configuration and molecular conformation of the substrate has been altered. Bulking the cell wall by reacting chemicals to cellulose, the hemicelluloses and lignin would reduce the tendency of wood to swell with changes in moisture because the wood would already be in a partially, if not completely, swollen state.
If the hydroxyl groups on the cel wall polymer are esterified with acetic anhydride, both dimensional stability and resistance to biological attack can be achieved.
Although several methods of acetylating wood designed for stabilizing the dimensions or for biological resistance of wood and other cellulosic materials have been suggested, all have failed in achieving any real commercial significance. In general, the prior art methods suffer from one or more of the following disadvantages: the process is too cumbersome or time consuming, the process is too complicated, the process is excessively expensive, the process requires ovendry wood, or the process imparts undesirable properties to the products.
Wood flour or sawdust was acetylated by W. Fuchs (Ber. 61B: 94B (1928)) and O. Horn (Ber. 61B: 2542 (1928)) using acetic anhydride containing 0.25 percent sulfuric acid, while H. Suida and H. Titsch (Ber. 61B: 1599 (1928)) used acetic anhydride/pyridine mixtures or acetic anhydride alone, and the process involved treatments for 15 hours. In 1930, H. Friese (Ber. 63B: 1902) acetylated powdered wood with mixtures of acetic acid and acetic anhydride catalyzed by sulfuric acid. H. Suida (Austrian Pat. No. 122,499 (1930)) reacted wood with acetic anhydride using a tertiary organic base as a catalyst.
W. B. Ridgway and H. T. Wallington (British Pat. No. 579,255 (1946)) acetylated wood, either veneer or ground, with acetic anhydride using a multivalent metal halid as catalyst. The preferred treatment was a mixture of acetic anhydride, acetic acid, and zinc chloride for 24 hours at 38 degrees to 50 degrees Centigrade.
A. J. Stamm and H. Tarkow (U.S. Pat. No. 2,417,995 (1947)) treated ovendry wood veneers with a moisture-free acetylation medium containing acetic anhydride mixed with other components such as a tertiary amine and acetone. The preferred treatments are carried out as a vapor-phase operation with a mixture of acetic anhydride and pyridine. That acetylation procedure has not had commercial acceptance because of certain disadvantages, such as: (1) pyridine forms complexes making recovery difficult; (2) if the reaction temperature is too high, the pyridine body darkens the wood; (3) if the reaction temperature is too low, the reaction period is relatively long; and (4) the various operations require a substantial amount of handling of noxious or flammable chemicals.
In 1963 I. G. Goldstein and J. W. Weaver (U.S. Pat. No. 3,094,431) described an acetylation procedure that eliminated the catalyst. Acetic anhydride was combined with xylene to acetylate wood at 105 degrees Centigrade and 150 to 170 psi pressure under vacuum. While the procedure eliminated the use of a catalyst, it introduced a volatile, flammable organic cosolvent that required special handling and complicated and excess reagent and byproduct recovery.
It has also been shown by Klinga and Tarkow (Tappi, the Journal, the Technical Association of the Pulp and Paper Industry, Vol. 49, No. 1, January 1966) that it is possible to obtain a stabilization of hardboard by "uncatalyzed vapor-phase acetylation," however, the board contained aluminum sulfate which could act as a catalyst. The necessary exposition time was, however, very long, "overnight heating was adopted."
U.S. Pat. No. 3,037,902 to Fahey discloses the acetylation of cellulose, not wood, at very high temperatures ranging from 135 to 280 degrees Centigrade.
U.S. Pat. No. 4,592,962 to Aoki dicloses a process for acetylating a wood material with an aqueous solution of an alkali metal acetate catalyst. Specifically, it discloses using a 5% solution of sodium acetate in a pretreatment, then drying, and finally acetylation.
U.S. Pat. No. 4,486,475 to Shutov et al. discloses a method of modifying wood with thermosetting resins.
U.S. Pat. No. 4,194,033 to Motai discloses a process of treating wood with alkylamine.
U.S. Pat. No. 4,127,686 to Motai discloses a process for treating wood with surface active agents.
U.S. Pat. No. 3,985,921 to Rowell et al. discloses reacting cellulosic materials with butylene oxide under mildly alkaline conditions to increase resistance to fungi while improving dimensional stability.
U.S. Pat. No. 3,894,839 to Marmer et al. discloses acetylation of materials using strong oxy acid and isopropenyl esters.
U.S. Pat. No. 3,649,341 to Tammela et al. discloses acetylation of cellulose using alkali metal acetate as a catalyst.
U.S. Pat. No. 3,094,431 to Goldstein et al. discloses the acetylation of wood using an organic cosolvent.
U.S. Pat. No. 2,417,995 to Stamm et al, discloses acetylation of wood but uses sodium acetate as a catalyst.
U.S. Pat. No. 2,031,973 to Mudge discloses impregnation of wood with mineral waxes.
As is evident from the foregoing, there are disadvantages connected to all the methods referred to. In fact, no method according to the prior art is suitable for acetylation on an industrial scale. Stabilization by acetylation has therefore been utilized in a very low degree in spite of the advantages the stabilized wood and wood products will have by use as a construction material. The high production cost and other drawbacks mentioned by the known methods have been a hindrance to their commercialization.
Thus, a primary object of this invention is to develop a process for acetylating lignocellulosic materials which does away with complex reaction mixtures and expensive pressure-treating equipment necessary with processes in the prior art.
A second object of this invention is a process which will increase dimensional stability and biological resistance to lignocellulosic materials.
Other objects and advantages will become apparent hereinafter from the description and drawing.